This proposal has two main objectives: a) to determine whether changing from one gait to another (walk to run to sprint in bipeds; walk to trot to gallop in quadrupeds) is a mechanism for maximizing muscular efficiency as running animals increase speed; and b) to determine the relationship between muscular activity and basal metabolism by studying the effect of equalizing daily activity in species with widely divergent basal metabolic rates. We propose to use both electromyographic measurements and estimations of glycogen depletion in muscle fibers to determine which muscles, and which fibers within a muscle, are active as animals run at three speeds in each gait in rats, dogs, quail and macaques. We propose to measure oxygen consumption of whole animals over 48-hour periods, metabolic enzyme potential and protein turnover in muscle, and muscle substrate resynthesis in lizard, Tegu, opossum and rat during normal activity and during conditions where activity is equalized. These animals have basal metabolic rates which normally differ by more than twenty-fold and similar incremental energy expenditures for equivalent locomotion.